Nematic liquid crystal devices are known. They are normally used for display units and are essentially based on the so-called electrically controlled birefringence (ECB), i.e. on the property of a suitably orientated electric field to rotate the nematic molecule, which has dielectric anisotrophy. In practice, the light which traverses a nematic crystal encounters a different refractive index according to the applied field. If the crystal is uniformly orientated and placed between mutually orthogonal polarizers, the polarized light which traverses it undergoes an ON-OFF effect as a function of the applied electric field.
However, these known nematic liquid crystal devices do not allow continuous polarization variation, but only sudden variations, which are of limited amplitude. Moreover, they do not allow the formation of coloured display units.
In addition to nematic liquid crystals, cholesteric liquid crystals are also known, these have a helical order in space in addition to an oriented order in the plane. At present, cholesteric liquid crystals are utilized for their property of continuously varying their colour as a function of temperature. For this reason they have been used primarily in the field of temperature measurement.